Solid state starters/controllers have found widespread use for controlling application of power to an AC induction motor. The conventional starter/controller, referred to hereinafter as simply a starter or a controller, uses solid state switches for controlling application of AC line voltage to the motor. The switches may be thyristors such as silicon controlled rectifiers (SCRs) or triacs.
One application for a motor controller is as an elevator starter. The elevator starter may be used to drive a pump for an hydraulic elevator. Each time movement of an elevator car is commanded, then the starter must start the motor until it reaches operating speed and then operate in a run mode. Such a starter may only be used for the up direction as gravity may be used for the down direction.
A conventional motor controller system may use current transformers for sensing current in the SCRs, generally representing current of each motor winding. The SCRs can be wired with the motor windings in a delta configuration or in a wye configuration. In a delta configuration, a shorted SCR will cause current to flow in the motor in a non-run mode, namely when the SCR is on but should be off, the motor is connected to the motor controller system and voltage is present. The current transformers can sense this current. However, with in-line wiring as in a wye configuration a shorted SCR does not produce current flow. Instead, there must be two shorted SCRs to have current flow in a non-run mode.
Typically, a fault contactor includes fault contacts wired in series with the SCRs and can be used for isolation. Another known method for detecting shorted SCRs is to sense for a low voltage across the SCR in a non-run mode. However, if a fault contact is opened or there is an open motor winding, then these conditions will also show a low voltage across the SCRs. This can cause such a motor controller to sense a shorted SCR when in fact the SCR is not shorted. This can result in unnecessary service calls.
The present invention is directed to solving one or more of the problems discussed above, in a novel and simple manner.